The demand for electricity continues to grow globally. In order to keep stride with the growing demand, coal is being looked to as a source for its generation. At present, burning coal produces some 50% of the electricity generated in the United States. The burning of coal in power generation plants results in the release of energy, as well as the production of solid waste such as bottom and fly ash, and flue gas emissions into the environment. Emissions Standards, as articulated in The Clean Air Act Amendments of 1990 as established by the U.S. Environmental Protection Agency (EPA), requires the assessment of hazardous air pollutants from utility power plants.
The primary gas emissions are criteria pollutants (e.g. sulfur dioxide, nitrogen dioxides, particulate material, and carbon monoxide). About two thirds of all sulfur dioxide and a quarter of the nitrogen dioxide in the atmosphere is attributable to electric power generation achieved by burning coal and other fuels.
Secondary emissions depend on the type of coal or fuel being combusted but include as examples mercury, selenium, arsenic, and boron. Selenium chemistry is similar to sulfur chemistry where selenium exists in flue gas as selenium dioxide and immediately converts to the selenite ion upon absorption into the liquid stream within the flue gas wet scrubber process. While certain chemistries have been known to successfully capture heavy metal cations, such chemistries would not expect to be effective at reducing selenium concentrations in liquid waste generated from flue gas wet scrubber processes.
Furthermore, because of the presence of high concentrations of halogens in the flue gas that is transferred to the scrubber liquor, flue gas wet scrubber processes can fail from corrosion. Recently, a metallurgy designated as 2205 alloy stainless steel has become popular in the industry due to its low cost and high chloride resistance. Typically, these scrubbers will operate with 10 to 12,000 ppm chloride concentration in their liquors. Unfortunately, flue gas wet scrubbers constructed of 2205 alloy have experienced increased levels of pitting and localized corrosion, resulting in forced, unscheduled unit shutdowns and expensive repairs.
One potential solution to the corrosion problem that has been implemented commercially is the use of potential adjustment protection (“PAP”) systems. PAP systems apply an electrical potential across the metal surface in an effort to control and reduce corrosion. This method has been successful, but it requires the installation of electrodes into the scrubber and the constant application of electrical potential and current. It also suffers from being applicable to only wet surfaces within the scrubbers.
Another solution to the problem is the operation of flue gas wet scrubbers at less than 2000 ppm chloride concentrations. While this prolongs the time between failures, it does not completely halt corrosion. Additionally, this solution results in higher blowdown rates, translating into higher water usage, higher wastewater treatment costs, and overall higher operating costs.
Consequently, there remains a need for a technology that can cost-effectively remove selenium from flue gas in flue gas wet scrubber processes. Ideally, the technology would have the added benefit of minimizing corrosion in flue gas wet scrubber processes. The invention described below addresses these needs.